Flashing is used to prevent water from working under the roofing material in specific areas, such as valleys, around chimneys, vent pipes, skylights, and where roof planes change. Common flashing materials include lead, copper, aluminum and galvanized steel.
Primary types of flashing include the following:
Valley flashing is used in an open valley as the most visible flashing on a roof and is often a source of leaks. The most common valley flashing is called W-flashing for its raised center to prevent water from sloshing back and forth and under shingles. It comes in 10-foot lengths and ranges from 16 to 24 inches wide. Problems can develop in regions of extreme temperature variations if the valley flashing is nailed so tightly to the roofing deck that it cannot expand without buckling. In moderate climates this is not a problem. Some roofers nail only the top edge of the flashing, then apply tabs over the sides that allow it to expand and contract without buckling. Shingles that overlap the edges of the flashing must be trimmed back far enough that runoff water will not be constricted and thus diverted under the shingles.
Plumbing vent flashing uses vent pipes to penetrate all roofs and prevent rain from running down between the pipe and the opening cut in the roof. The most common type is a square of galvanized tin with a metal cone in the middle that is rimmed with a rubber gasket. This type of flashing is sold by the size of the vent pipe and the angle of the roof. The cone, which is angled according to the approximate pitch of your roof, slips over the vent pipe and the rubber gasket seals around it. Roofing material is then applied over the flashing.
Lead flashing is most commonly used on tile roofs, both concrete and clay, where the flashing must be formed around the tile. One example is plumbing vent flashing. First, a hole is cut in the tile that fits over the pipe and that tile is fitted in place. The hole is filled with concrete around the pipe. Next, the flashing and cone are slipped over the pipe and the soft lead flashing is molded to fit the tile's surface shape. The next course of tile covers the top edge of the flashing so water cannot get beneath it.
Step flashing is standard where the flashing must be carried up the roof, like a series of steps. This type is regularly used next to chimneys or beside dormer walls that intersect with a sloping roof. The step flashing is aluminum or galvanized tin about 6 or 8 inches wide and a foot long. It is bent in the middle so that one end lies flat on the roof and the other fits snugly against the vertical wall and is covered by the siding. For chimney flashing, another piece of flashing called counter flashing, is also used. The top of the counter flashing is embedded in the mortar between the bricks and the remainder is bent down over the step flashing.